1. Field of the Invention
The present invention relates generally to roller thrust bearings including needle roller thrust bearings and particularly to thrust bearings arranged in rotative portions of electronic components of automobile transmissions (manual and automatic), transfers or compressors for automobile air conditioners and the like to support thrust loads exerted on the rotative portions.
2. Description of the Background Art
With reference to FIGS. 6A and 6B, a roller thrust bearing 1 includes a plurality of circumferentially, radially equally arranged rollers 2, a cage 3 formed in a circular ring and holding the plurality of rollers 2 rotatably, and outer and inner rings 4 and 5 serving as a roller bearing ring sandwiching and thus supporting the plurality of rollers 2 on their opposite sides. Outer and inner rings 4 and 5 are each formed in a circle formed of a metal plate having a sufficient hardness. Outer ring 4 has a circular ring surface 6 of its own and inner ring 5 has a circular ring surface of its own.
A roller thrust bearing is a bearing having a variety of advantages. For example it can have a simple structure and still provide high load capacity and high rigidity. Furthermore it can be structured without the aforementioned roller bearing ring and formed of a cage and a roller alone. In that case, a shaft, a housing or the like that has a counterpart rolling contact surface to the roller is provided with roughness, hardness or any other similar function comparable to the bearing's ring surface, and using the shaft, the housing or the like as a ring surface allows the bearing to dispense with a roller bearing ring and thus be compact. In particular, a roller thrust bearing using a needle roller, or a needle roller thrust bearing, can be decreased in height, as seen in cross section, and thus suitable for compact mechanical designs.
A main application of a needle roller thrust bearing, a typical example roller thrust bearings, is a compressor of an automobile air conditioner. It includes a variety of types. For example, FIG. 7 shows a double-inclined-plate compressor in which a 2-side inclined plate 9 secured to an input rotary shaft 8 allows a piston 10 to reciprocate. As another example, FIG. 8 shows a single-inclined-plate compressor in which a 1-side inclined plate 12 secured to an input rotary shaft 11 allows a piston 14 to reciprocate via a rod 13. As still another example, FIG. 9 shows a variable-capacity, single-inclined-plate compressor in which an inclined plate 16 secured variably in angle to an input rotary shaft 15 allows a piston 18 to reciprocate via a rod 17. The above types all have a rotative portion incorporating a roller bearing.
Note that the FIGS. 7-9 compressors do not necessarily belong to conventional art. They should be considered an apparatus which will further be improved in future for example through further development of a roller thrust bearing contemplated by the present invention. In other words, they are an apparatus having its internal components being improved in structure, material and the like.
In a typical bearing, such as ball bearings, a differential slip is introduced between a rolling element and a roller bearing ring. The differential slip of such bearings basically depends on a difference in circumferential velocity in a contact surface of the rolling element and the roller bearing ring. More specifically, a ball bearing provides a point contact and hence a small contact area. As such, within a contact surface their difference in circumferential velocity and hence their differential slip would be small.
In contrast, in a roller thrust bearing, as basically structured, a rolling element in the form of a cylindrical roller is arranged on a roller bearing ring having a planar ring surface, the roller and the roller bearing ring contact each other in a line, and the bearing's center of rotation matches the roller's center of revolution. In that case, the circumferential velocity on a rolling contact surface of the roller is the same velocity, whereas the roller bearing ring contacting the roller as it rotates provides a circumferential velocity increasing, as seen radially outward, away from the bearing's center of rotation (in proportion to the roller bearing ring's radius of rotation). Therefore the roller and roller bearing ring's difference in circumferential velocity has a maximal value at the roller's opposite ends. In theory, only on a pitch circle of the bearing a no-slip rolling movement is provided. From a point on a pitch circle of the roller toward the opposite ends of the roller, the difference in circumferential velocity between the roller and the roller bearing ring increases, and the differential slip increases, in proportion to the roller's length.
As aforementioned, differential slip internal to a roller thrust bearing is greater than those of bearings of different types. As such, a differential slip of the roller and the roller bearing ring disadvantageously causes generation of a stress at the roller's edge that is introduced between the roller bearing ring and the roller, which causes flaking of an edge of a rolling and running portion of the roller bearing ring that starts at a surface thereof.
The above disadvantage has conventionally been addressed by a bearing having a roller reduced in length to reduce relative slippabiliy of the roller's end surface or a bearing having two rollers arranged in each pocket of the cage, i.e., in multiple rows. Furthermore, the disadvantageous stress or load on the roller's end overcome for example by using a crowned roller (Japanese patent Laying-open No. 9-14131).
There is a demand for a roller thrust bearing reduced in size to save energy and space and have a reduce weight (a reduced friction loss). Accordingly, the bearing in use encounters an increasingly severe condition in terms of load capacity. As aforementioned, in a roller thrust bearing a differential slip of a roller and a roller bearing ring increases friction loss and wear that are caused at a contact surface. Furthermore the contact surface's oil film formation is impaired, and at the roller's edge an edge stress between the roller bearing ring and the roller itself is readily introduced, which causes flaking of an edge of a rolling and running portion of the roller bearing ring that starts at a surface thereof. The differential slip may be reduced simply by reducing the roller in length. This, however, reduces the roller's contact area and thus provides increased contact pressure. Contact pressure at the contact surface thus increases, which results in impaired oil film formation and the exfoliation (flaking) for example disadvantageously occurs at the roller's radically outer surface.
A compressor uses a bearing having an interior with the bearing's lubricant mixed with a coolant. Furthermore, as the compressor contracts and expands, the lubricant is liquefied and vaporized repeatedly. The lubricant is thus disadvantageously reduced in amount. As such, a worse oil film is formed than when a typical machine operating lubricant is used, and the bearing flakes earlier than expected.
Furthermore to address global warming and other similar environmental issues a compressor for an air conditioner uses HCFC134a or other similar alternative flon as its coolant. These alternative flons are considered as being poorer in self-lubricity than conventionally used coolants. With the alternative flon dissolved and thus mixed, the lubricant for example has a reduced kinematic viscosity, and the bearing has a significantly impaired oil film formation. As such, the bearing would have a rolling element, a roller bearing ring and the like flaking, wearing or having a surface similarly damaged, resulting in reduced longevity.
This may be handled by improving the lubricant. However, because of its chemistry with the coolant, the lubricant can only be selected from a limited range and a significantly enhanced ability to form an oil film cannot be expected. Increasing the amount of the lubricant in the coolant to enhance lubricity decreases the amount of the coolant and thus impairs the compressor's cooling ability.
As used in an air conditioner's compressor, a needle roller thrust bearing receives a thrust load offset from the center of rotation and rotates rapidly at no less than approximately 8,000 rpm. Such a rotation rate and a load are also severer conditions, increasing a defect indicating damage of a surface of the bearing that is attributed to a differential slip.